The fuel cell has tremendous promise as a future power system due to its low pollution, low noise, and high efficiency. Fuel cells convert a fuel and an oxidizing agent, spatially separated from each other, into electricity, heat and water at two electrodes (i.e., the anode (−) and the cathode(+)). Fuel cells are also suitable for use as energy converters in transportation and portable energy devices because of their compact structural design, power density, high efficiency, and little or no emission.
A typical fuel cell unit consists of a stack of individual fuel cells; the actual number of cells depends on the power needs. With reference to FIG. 1, the fuel cell typically includes flow field bipolar plates (stippled) (graphite composite or metal plates, as examples) that are stacked one on top of the other. The plates may include various surface flow field channels and/or orifices to, for example, route the reactants and products through the fuel cell stack (arrows). They also act as current collectors. A polymer electrolyte membrane (PEM), often called a proton exchange membrane, which permits only protons to pass between an anode and a cathode of the fuel cell, is sandwiched between each anode and cathode flow field plate. Electrically conductive gas diffusion layers (GDLs) are located on each side of the PEM and the bipolar plate to act as a gas diffusion media and in some cases to provide a support for the fuel cell catalysts. In this manner, reactant gases from each side of the PEM may pass along the flow field channels and diffuse through the GDLs to reach the PEM. The PEM and its adjacent pair of catalyst layers are often referred to as a membrane electrode assembly (MEA). Typically MEA is constructed by painting a carbon GDL particle with platinum catalyst on the PEM.
In a hydrogen fuel cell, hydrogen or a hydrogen-rich gas/liquid is used as the fuel and oxygen or air as the oxidizing agent. The bipolar plates, which form the flow field for the reactant and product, and house the coolant channels, collect the current produced in the cell and transmit it to the external circuit (or to the next cell in the stack).
Currently available GDLs are comprised of carbon fiber paper or carbon fiber cloth, as shown in FIG. 2. However, the carbon fiber gas diffusion media do not meet long-term requirements for fuel cell performance, durability, and cost. For example, carbon fiber GDLs suffer from difficulty in controlling the pore parameters and the pore sizes distribute randomly. In addition, the carbon fiber GDLs are generally made hydrophobic by treating with PTFE (i.e., Teflon), which increases their weight from 5 to 30%. These treatments also reduce both electronic and thermal conductivity. During operation the GDL is normally under compressive stress, which reduces thickness, and decreases the porosity and permeability of carbon paper or carbon cloth by up to 50%. These shortcomings limit the durability of the GDL, and the fuel cell.
The GDL is very important to the overall operation of the fuel cell and effective diffusion media promotes a uniform current distribution at the adjacent catalyst layer. More particularly, the GDL performs the following key functions: (1) provides reactant gas access from flow-field channels to catalyst layers; (2) provides passage for removal of product water from catalyst-layer area to flow-field channels; (3) provides electronic conductivity from bipolar plates to catalyst layers; (4) provides for efficient heat removal from MEA to bipolar plates where coolant channels are located; and (5) provides mechanical support to the MEA in case of reactant pressure difference between the anode and cathode gas channels, maintain good contact (i.e., good electrical and thermal conductivity) with the catalyst layer, and not compress into the channels resulting in blocked flow and high channel pressure drops.
Many fuel cell membranes need to be maintained in a hydrated state to function properly. It is especially important to maintain membrane hydration while a fuel cell is operated, since proton conductivity is increased with membrane hydration. However, in some cases the reactants flowing through a fuel cell may become subsaturated with water as they are heated by the fuel cell reaction, and may thus tend to dry out the PEM; resulting in permanent damage to the PEM. Dry areas can also eventually spread until a PEM no longer functions at all. For this reason, fuel cell reactants in such systems are generally saturated with water vapor before they are supplied to the fuel cell.
It should also be noted that the fuel cell reaction produces product water besides heat at the cathode side of the PEM. Excess product water (in excess of the amount required to keep the reactants and PEM saturated) must be removed in order to prevent blockage of the flow field channels and flooding on gas diffusion layers and the catalyst layer that could prevent reactant gasses from reaching reaction sites. Various water management methods have been used to address these issues. For example, a coolant associated with a fuel cell may be circulated to control the temperature rise of the reactants flowing through the flow field channels such that the reactant streams remain saturated as they remove product water formed in the cells. In some cases, subsaturated reactants may be flowed through a fuel cell for a period to dry out the cell when it appears “flooded” by excess water. Water tends to diffuse rapidly through most PEMs, so that a subsaturated reactant flow on one side of the PEM can serve to remove excess water from both sides of the PEM. The flow rate of reactant flowed through the fuel cells relative to the electrical load on the cells (also referred to as reactant stoichiometry or “stoich”) may also be increased to help remove water from the cells.
Interestingly, the pore-size distribution of the GDL plays an important role in improving cell performance and minimizing the effect of mass-transport limitation. Thus, the issues of flooding and mass-transport limitation under steady-state and transient (e.g., start-up) conditions demand careful GDL design. Currently, it widely believed that if the pores of the GDL are aligned in a straight configuration (as compared to mesh-like interconnectivity) the water would freely drain out once water flow had been initiated.
Thus, there is a continuing need to improve fuel cell durability, design, and efficiency in a robust, cost-effective manner.